There are two factors that come into play when shaping hammers: the resilience and elasticity of the hammers at the points that they hit the string, and the shape of the hammer as it leaves the string. Overall, it is the shape of the string when the hammer leaves it.
As hammers get worn, the strike point flattens and hardens, so the shape of the string is more angular, and in fact, angular in the shape of the hammer at the strike point, so at both ends of the strike point. This angularity persists until the stiffness of the wire smoothes it out, but it affects the attack.
If the hammers are too hard at the tip, you have the same affect, except with just an angle at the tip. This can happen from over-juicing the tip of the hammer, while over-juicing up the sides will give you the effect of a flat strike point. You should be sparing when juicing hammers. If should not saturate the hammer near the top. That way, if the sound is too harsh after juicing, you can sand away the lacquer until it sounds nice.
Of course, when you sand, you want to make sure that all the strings strike evenly, so you do not have one string making one sound while the others are making a different sound.